1. Field of the Invention
The present invention relates to an equalizer with a phase-locked loop which detects and eliminates a phase shift from a received signal which is subjected to a frequency offset or a phase variation generated at the beginning of a burst transmission and more particularly, to a delayed decision feedback sequence estimator with a phase-locked loop which can compensate a phase shift of a received signal which is terribly deteriorated by a transmission distortion.
2. Description of the Prior Art
A delayed decision feedback sequence estimation receiver as a first prior art which has been disclosed in JPA 11-8573 is shown in FIG. 1.
Referring to FIG. 1, transmission line characteristic detector 41 detects discrete impulse responses from a received signal in a preamble, i.e. a training signal of an M-sequence. Absolute value calculator 42 calculates an amplitude of each impulse response. Accumulator 43 sets three regions, i.e. a maximum likelihood sequence estimation region, a decision feedback equalization region, and a non-estimation region, and obtains a sum p of amplitudes of impulse responses in the maximum likelihood sequence estimation region, a sum q of amplitudes of impulse responses in the decision feedback equalization region, and a sum r of amplitudes of impulse responses in the non-estimation region each time an amplitude of a succeeding impulse response is inputted. Maximum detector 44 calculates a value of p/(r+xcex1q) for each impulse response group among shifted groups and outputs a signal which indicates the impulse response group which maximize the value. Delayed decision feedback sequence estimator 45 estimates the maximum likelihood sequence from the received signal by using the impulse responses selected from the impulse responses inputted from transmission line characteristic detector 41 by the signal from maximum detector 44.
Next, the calculation in maximum detector 44 which determines the optimum regions of impulse responses will be explained. Components in the decision feedback equalization region are ideally cancelled by feedback and do not contribute to improvement or deterioration of the estimation capability of delayed decision feedback sequence estimator 45. Therefore, the estimation capability is determined by the ratio of p/r, i.e. the ratio of the sum p of amplitudes of impulse responses in the maximum likelihood sequence estimation region and the sum r of amplitudes of impulse responses in the non-estimation region, and the greater the ratio of p/r, the higher the estimation capability.
However, because of quantization errors and the like, components in the decision feedback equalization region are not canceled completely and remain as a distortion. Therefore, the greater the ratio of p/(r+xcex1q), i.e. the ratio of the sum r of amplitudes of impulse responses in the maximum likelihood sequence estimation region and the sum of the sum r of amplitudes of impulse responses in the non-estimation region and the weighted sum xcex1q of amplitudes of impulse responses in the decision feedback equalization region, the higher the estimation capability. That is, the impulse response group which maximizes the ratio of p/(r+xcex1q) indicates the optimum regions.
A phase-locked loop circuit using equalizer as a second prior art which as been disclosed in JPA 10-327204 is shown in FIG. 2.
Impulse response detector 33 is similar to transmission line characteristic detector 41 of the first prior art. Delayed decision feedback sequence estimator 32 is similar to delayed decision feedback sequence estimator 45 of the first prior art. Region designator 34 is similar to a group of absolute value calculator 42, accumulator 43, and maximum detector 44, each of which is of the prior art. Replica generator 35 convolutes impulse responses designated by region designator 34 among impulse responses of a transmission line obtained by impulse response detector 33 in a preamble period with a sequence signal estimated in delayed decision feedback sequence estimator 32 in order to generate a replica of a received signal. Delay circuit 36 delays the received signal which has been rotated in phase in phase rotator 31 to compensate the delay which is generated in delayed decision feedback sequence estimator 32. Thus, an output from delay circuit 36 coincides with an output from replica generator 35. Phase detector 37 detects a phase difference between the output from replica generator 35 and the output from delay circuit 36. Here, if the received signal has a frequency offset, the phase difference between the transmission side and the reception side varies with a laps of time, and the phase difference detected by phase detector 37 varies with a laps of time. That is, because replica generator 35 outputs a signal with no phase variation as long as delayed decision feedback sequence estimator 32 does not cause errors as the impulse response of a transmission line used in replica generator 35 is constant, while delay circuit 36 outputs a signal with a phase variation, phase detector 37 detects a phase difference between the signals.
An output signal from phase detector 37 is in bandwidth restricted by filter 38 and inputted to VCO (Voltage Controlled Oscillator) 39. Phase rotator 31 rotates a phase of a received signal by using an output of VCO 39 to reduce the phase difference detected by phase detector 37, thereby absorbing a phase variation due to a frequency offset or the like.
The second prior art has a disadvantage that a replica generated in replica generator 35 may deteriorate in precision for some impulse responses of a transmission line. This disadvantage when a delayed decision feedback sequence estimator is used as a signal estimator will be explained below.
FIG. 3 shows an example of an impulse response and regions. In this example, the symbol length of maximum likelihood sequence estimation region 53 is 4, and the symbol length of decision feedback equalization region 54 is 3. There are direct wave 51 and delayed wave 52 delayed from direct wave by 5T (T: symbol period), thereby constituting a two-wave model. In practical circumstances, there is a case that a level of direct wave 51 is extremely lowered because of fading. In such a case, as a result of calculation of regions as explained above, maximum likelihood sequence estimation region 53 includes a delayed wave 52 and both of maximum likelihood sequence estimation region 53 and a decision feedback equalization region 54 do not include direct wave 51.
A delayed decision feedback sequence estimator has such a feature that a maximum likelihood sequence estimation region and a decision feedback equalization region 53 are determined in such a way that a direct wave or one or more delayed waves are not included in both of a maximum likelihood sequence estimation region and a decision feedback equalization region though the direct wave and all the delayed waves may be included in any of maximum likelihood sequence estimation region 53 and decision feedback equalization region 54.
Delayed decision feedback sequence estimator 32 obtains a better estimation characteristics when executing a maximum likelihood estimation using a delayed wave high in level than using a direct wave low in level as shown in FIG. 3. However, when obtaining a phase difference between a replica and a received signal, the replica does not include a direct wave which falls out of a maximum likelihood sequence estimation region and out of a decision feedback equalization region. Therefore, the phase difference includes an error composed of the direct wave. This error is not caused by a phase variation but the error is fedback to phase rotator 31 as if there is a phase variation, thereby deteriorating a estimation characteristics.
In order to overcome the aforementioned disadvantages, the present invention has been made and accordingly, has an object to improve estimation characteristics in an equalizer, especially a delayed decision feedback sequence estimator, by operating in high-accuracy a phase-locked loop which compensates a phase variation due to a frequency offset or the like and is included in a receiver which estimates a signal on the basis of a received signal which has been extremely distorted in a transmission line by using the equalizer.
According to an aspect of the present invention, there is provided an equalizer with a phase-locked loop comprising: a phase rotator which rotates a phase of a received signal to output a rotated signal; an impulse response detector which detects impulse responses of a transmission line through which the received signal has been transmitted on the basis of the rotated signal; a first region designator which designates a first region on the basis of the impulse responses; a second region designator which designates a second region on the basis of the impulse responses; an equalizer which estimates a sequence on the basis of the rotated signal by using the impulse responses in the first region; a replica generator which generates a replica of the received signal on the basis of the sequence by using the impulse responses in the second region; a phase detector which detects a phase difference between the replica and the rotated signal; and means for controlling the phase rotator on the basis of the phase difference to decrease the phase difference.
The equalizer with a phase-locked loop may further comprise: a variable delay circuit which delays the rotated signal to be supplied to the phase detector by a period determined by the first region and the second region.
The equalizer may be a delayed decision feedback sequence estimator; and the first region may be divided into a third region and a four region.
The third region and the four region may maximize P/(R+xcex1Q), wherein P stands for a sum of powers of the impulse responses in the third region, Q stands for a sum of powers of the impulse responses in the fourth region, R stands for a sum of powers of the impulse responses in a region other than the third and fourth regions, and xcex1 stands for a coefficient.
The second region may maximize (P+Q)/R, wherein P stands for a sum of powers of the impulse responses in the third region, Q stands for a sum of powers of the impulse responses in the fourth region, and R stands for a sum of powers of the impulse responses in a region other than the third and fourth regions.
The impulse response detector may detect the impulse responses on the basis of the rotated signal in a preamble.
The means for controlling may comprise: a filter which restricts a bandwidth of the phase difference; and a voltage controlled oscillator which generates a sinusoidal wave to be supplied to the phase rotator in response to the phase difference restricted in bandwidth.